A semiconductor chip or die (e.g., an image sensor chip) is typically fabricated on a single semiconductor wafer along with hundreds, and in some cases thousands, of copies of the same die. Cutting is needed to separate individual dies from a semiconductor wafer. Such cutting is known as dicing or wafer dicing and is typically performed using a die saw. A common die saw used for dicing is a diamond saw. Conventionally, cuts are made along non-functional areas of semiconductor material, known as scribe lines, which separate the dies on the wafer from each other. Using a diamond saw introduces mechanical stress to the semiconductor wafer and can result in cracking at the die edges thereby compromising the integrity and reliability of integrated circuit (IC) devices present on the die. One alternative to using a diamond saw is laser scribing. Laser scribing involves scanning a laser beam over the scribe lines of a semiconductor wafer; however, laser scribing typically provides a low throughput and is expensive.
Typically, scribing or cutting is carried out on the active side of the semiconductor wafers where the IC devices are formed and the scribe lines are defined. Conventional scribe line areas do not have circuit elements of the die areas, and because each die is an independent device, metal features for interconnect wiring conductors are also confined to the die areas and do not extend into or across the scribe lines where the die saw will cut through the wiring layers. Some wafer-level reliability and functionality test pads, however, can be located in the scribe line areas to facilitate wafer-level testing. In such wafers, the scribing or cutting across the test pads generally results in severe dielectric peeling and cracking. These de-laminations become sources of defects that detrimentally affect the integrity and reliability of the diced chips.
Further, an increasing demand for image sensors having faster processing speeds and better image quality has led to smaller pixel cells sizes and smaller photosensitive regions or photodiodes. Advances in semiconductor technology including the use of low-k or extremely low-k (ELK) dielectric materials to reduce cross coupling and parasitic capacitance between metal layers are employed to match the pace of decreasing image sensor chip size. Such low-k and ELK dielectrics, however, are brittle due to their porous nature subsequently making image sensors with such materials prone to peeling and cracking when diamond saws are used to separate dies on a semiconductor wafer.
Another conventional method of increasing the size of a photodiode in a pixel cell is to use a backside illuminated (“BSI”) image sensor. BSI image sensors typically include a pixel array fabricated on the frontside of the semiconductor wafer, but receive light through a back surface of the image sensor. During fabrication of a BSI image sensor, the image sensor chip or device is first fabricated on a semiconductor wafer, and when the necessary elements have been formed in or on the wafer, the device wafer is bonded to a carrier wafer for further processing. Due to the tradeoff of parameters such as bonding strength and wafer distortion, bonding strength can be maximized, but this can cause weakness at the bonding interface. This combination of low-k or ELK dielectrics in the device wafer and weak bonding interface can increase the occurrence of peeling and cracking when a diamond saw is used to dice BSI dies from the combined wafer. Thus, there is a need for improved scribe line structures in the semiconductor industry.